The Seasonality of Serum Insulin Concentrations in Equids and the Association With Breed, Age, and Sex

Current laboratory reference values for serum insulin concentrations do not account for seasonal fluctuations and differences associated with breed, sex, and age.

We hypothesized that serum insulin concentrations would differ with the season, breed, sex, and age.

Laboratory records from 21 236 cases.

Cases were included where basal serum insulin concentration (INS) or serum insulin concentration after administration of Karo light syrup (INS_pk) was measured, along with plasma glucose, breed, age, and sex. Multivariate analysis was used to investigate a seasonal effect on serum insulin concentrations, alongside the effect of breed, age, and sex.

Basal serum insulin concentration in winter was significantly higher than in the other seasons (p < 0.001). Serum insulin concentration following administration of Karo light syrup in winter was significantly higher than in summer and fall (p < 0.001). The breed effect was assessed in the 9 most prevalent breeds and the donkey. Shetland ponies had significantly higher INS and INS_pk than all breeds except Welsh ponies(p < 0.01). Welsh ponies had significantly higher INS than all other breeds except Shetland ponies (p < 0.01). Welsh ponies had significantly higher INS_pk than all breeds (all p < 0.001), except Arabians, New Forest ponies, and Shetland ponies. Females had significantly higher INS than males (p < 0.001) and there was a positive and significant association between age and INS (est = 0.02; SE = 0.002, p < 0.001).

Insulin concentrations are influenced by season, breed, age, and sex. This information is essential for better understanding and management of insulin dysregulation.